non-linear scaling of STL-files along the Z-Axis
You would need to import the file first. If this is only about different sized separate objects, that would not be very difficult. You would just need to loop through all the objects in your scene, check their size and scale them accordingly. Could look something like this: 

import bpy
from mathutils import Vector

for o in bpy.context.scene.objects: #bpy.context.view_layer.objects in 2.80
    if o.dimensions[2] &gt;= 1: # change the 1 to anything (dimensions[2] is Z)
        scale = Vector((2,2,2)) # change the scale to anything
        o.scale = [a*b for a,b in zip(o.scale, scale)]
    else: 
        scale = Vector((3,3,3)) # change the scale to anything
        o.scale = [a*b for a,b in zip(o.scale, scale)]